Synthetic images of different kinds have during a long time been used for many different purposes. One typical example of applications of synthetic images is as authentication purposes. A synthetic image is given different properties that are difficult to forge, but which are still easily noticed by the viewer's eyes and/or a simple assisting device. A very much used example of a system that has the ability to provide such synthetic images is a so-called Moiré device. In the two-dimensional version of such a device, a large multitude of small focussing elements are arranged in an array. An image plane is provided with optically distinguishable icons or geometrical features which are imaged through a respective focussing element towards a viewer's eyes, where the human brain creates a composite synthetic image.
Typical examples of prior-art systems that have been used or can be used for e.g. authentication purposes can e.g. be found in the published international patent applications WO 03/061983 or WO 2009/085003.
The published US patent application US 2009/0102179 A1 discloses a counterfeit proof label having an optically concealed progressive shifting security safety symbol for quick visual identification utilizing a mobile phone for online verification. In the published International Patent Application WO 2008/008635 A2 a micro-optic security and image presentation system is disclosed. In the published US patent application US 2009/0140131 A1 an image input apparatus, a photodetection apparatus and an image synthesis method are disclosed.
An important benefit for using synthetic images as authentication devices is that they are difficult to copy. A problem that has been discussed for a while is the problem for an un-trained viewer to be able to distinguish between a correct image and a forged one. The properties of the image have to be very distinct and easily distinguishable from different kinds of forged images. It can for instance be difficult for an untrained viewer to distinguish an image with a true three-dimensional appearance from an image having certain 3D-like properties. A request is thus to provide optical devices producing images than may be influenced to give large and easily observable changes when handled according to a predetermined treatment.